Single Semiconductor Nanocrystals under Compressive Stress

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Letter pubs.acs.org/NanoLett

Single Semiconductor Nanocrystals under Compressive Stress: Reversible Tuning of the Emission Energy Tobias Fischer, Sven Stöttinger, Gerald Hinze, Anne Bottin, Nan Hu, and Thomas Basché* Institute of Physical Chemistry, Johannes Gutenberg-University, Duesbergweg 10-14, 55128 Mainz, Germany S Supporting Information *

ABSTRACT: The photoluminescence of individual CdSe/CdS/ZnS core/shell nanocrystals has been investigated under external forces. After mutual alignment of a correlative atomic force and confocal microscope, individual particles were colocalized and exposed to a series of force cycles by using the tip of the AFM cantilever as a nanoscale piston. Thus, force-dependent changes of photophysical properties could be tracked on a single particle level. Remarkably, individual nanocrystals either shifted to higher or to lower emission energies with no indications of multiple emission lines under applied force. The direction and magnitude of these reversible spectral shifts depend on the orientation of nanocrystal axes relative to the external anisotropic force. Maximum pressures derived from the applied forces within a simple contact-mechanical model lie in the GPa range, comparable to values typically emerging in diamond anvil cells. Average spectral shift parameters of −3.5 meV/GPa and 3.0 meV/GPa are found for red- and blue-shifting species, respectively. Our results clearly demonstrate that the emission energy of single nanocrystals can be reversibly tuned over an appreciable wavelength range without degradation of their performance which appears as a promising feature with respect to tunable single photon sources or the creation of coherently coupled particle dimers. KEYWORDS: Core/shell nanocrystals, correlative microscopy, atomic force microscopy, confocal fluorescence microscopy

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a nominally nonhydrostatic setting in a diamond-anvil cell. Besides these experimental studies, extensive theoretical research has been published recently, examining the influence of isotropic17,26−30 and anisotropic19 pressure/forces on the electronic transitions17,19,27−30 and vibrations26 of semiconducting NCs. Based on these findings, semiconductor NCs have been utilized to locally probe pressure and stress31−33 in polymeric matrices. So far single molecule/nanoparticle experiments with high pressures as typically attained in diamond-anvil cells (GPa) have remained elusive. Early attempts to measure pressure shifts of single molecules have relied on narrow zero-phonon lines at liquid helium temperatures which are sensitive to low pressures in the sub-MPa range.34−36 Recently, a novel design of a pressure cell integrating a high numerical aperture objective has been described, which allowed to study single molecules under pressures up to 20 MPa at room temperature.37 In our approach we employ the tip of an AFM cantilever as a nanoscale piston to apply compressive forces on single semiconductor NCs. As will be shown below, thereby directional (nonhydrostatic) pressures in the GPa regime could be realized. To study the optical response of an individual NC to the applied force, we used a setup for correlative atomic force and

olloidal semiconductor nanocrystals (NCs) are bright and robust light emitters which vastly benefit from their tunable spectroscopic properties.1−4 Due to the quantum size effect, emission spectra can be easily shifted by solely changing the size of the NCs.5−7 The additional growth of an inorganic semiconductor shell onto the core particles can significantly alter the optoelectronic properties of the resulting core/shell structures.6,8−10 Due to lattice mismatch between core and shell layers, NCs might be subject to internal stress which modifies the electronic states.11−13 Similarly, but less pronounced, organic capping agents typically used in liquid-phase synthesis and to stabilize the colloid have been discussed to induce tensile or compressive stress on the NCs.13 Recently, several studies have been performed on NCs to elucidate the influence of external pressure on the quantum states involved in the optical transitions.14−23 Most of these experiments have been conducted in a hydrostatic configuration; i.e., pressure was applied isotropically on NCs, typically resulting in a blue shift of the emission lines.15−18,22−25 Interestingly, the fact that NCs are not highly compressible has been exploited to separate stress effects on the optical band gap from particle size/volume effects.21 Under nonhydrostatic conditions, ensemble experiments on differently shaped CdSe/CdS NCs have shown emission line splitting.20 In these bulk studies, however, it could not be verified whether the line splitting arose due to the probing of different populations or was an intrinsic feature of each NC. Moreover, these studies indicated that it is difficult to avoid hydrostatic contributions in © 2017 American Chemical Society

Received: November 9, 2016 Revised: January 30, 2017 Published: February 2, 2017 1559

DOI: 10.1021/acs.nanolett.6b04689 Nano Lett. 2017, 17, 1559−1563

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Nano Letters confocal fluorescence microscopy which has been described recently.38 Briefly, it consists of a commercial atomic force microscope (AFM) (MFP-3D, Asylum research) mounted on top of a home-built confocal fluorescence microscope (CFM). The whole setup is isolated against vibrations (Mod 1, Halcyonics) and temperature stabilized within a closed, sound-proof box (see Figure S1). For AFM imaging and force transduction we used standard silicon tips (AC240TS, Olympus) with an average radius of ∼7 nm. Light from a pulsed laser diode (470 nm) was coupled into a microscope objective (Zeiss apochromate, NA = 1.4) to excite single NCs. Fluorescence emission of the particles was collected by the same objective and after division by a beam splitter imaged onto a spectrograph/CCD to record emission spectra and an APD to measure fluorescence intensities and decay times. Two batches of CdSe/CdS/ZnS core/shell NCs with average diameters of 9 and 7.3 nm have been prepared via standard SILAR route9 (for characterization see Figures S2 and S3). Since no principal difference between the two batches has been observed under applied force, the discussion does not distinguish between them. This type of NCs was chosen because of their size and their high emission yields, both properties being favorable for our single particle approach. Samples were obtained from a dilute solution of NCs in toluene by spin coating on standard microscope coverslips. The concentration was adjusted to ensure interparticle spacings larger than the spatial resolution of the optical microscope. Thus, under our experimental conditions we can safely exclude that any of the effects observed might have been caused by interactions between several NCs. The surface of the coverslips was made hydrophobic by using mercaptopropyltrimethoxysilane.39 While lowering its water content and potentially reducing the photo-oxidation probability of the NCs,40 the mercapto groups increased the affinity of the NCs to the surface. Thus, sticking of NCs to the AFM tip, which was observed occasionally, was less common. Given the size of the NCs and their favorable photophysical properties, the essential spatial alignment of both AFM and CFM described elsewhere38 turned out to be straightforward. In Figure 1 simultaneously recorded AFM and fluorescence images of single NCs are displayed. After colocalization of a single NC with both microscopes, a succession of multiple force cycles was applied while simultaneously recording fluorescence spectra and lifetimes. In the following we will focus the discussion on the spectral

shifts (see Figure 2a), which were the main topic of our investigation. Each force cycle consisted of 2−3 s force build-up

Figure 2. (a) Succeeding force cycles (red line) on a single nanocrystal are accompanied by reversible alterations of the emission spectra, upper panel. The black line represents the emission maximum. (b) Single force cycle with emission maximum. The spectral shifts follow the force, not the distance of the AFM tip. (c) Emission spectra with and without applied force.

period from a first contact of the AFM tip with the particle surface until a preset value of 3−5 nN was reached. Afterward the cantilever was kept at a constant height for ∼3 s, which usually corresponded to an almost constant force. Within 2−3 s the AFM tip was then retracted to its initial height above the NC. In Figure 2b, an example for a single force cycle is plotted. Note that simultaneously observed shifts in emission energies follow the applied force, rather than the distance between tip and NC. Typically, such a series of force cycles lasted 100−500 s until either fluorescence vanished or no further force induced changes of the spectra were detectable. In total 1948 cycles on 77 NCs have been measured. To quantify the impact of the applied force or the derived pressure on the NC emission energies, the emission maxima were determined by fitting the line profiles to a sum of two Gaussians, satisfactorily reproducing their nonsymmetric monomodal shapes. Typical spectra with and without force are shown in Figure 2c. A similar figure showing data from a red-shifting NC under force is drawn in the Supporting Information S4. In the discussion of ensemble experiments with CdSe/CdS NCs under nonhydrostatic pressure the question had been raised, whether red and blue peaks originated from different populations of NCs or several allowed transitions within individual particles.20 The single particle data presented here give a definite answer to this crucial issue. As demonstrated in Figure 2c, emission from a NC typically features a single emission line. In no case line splitting has been observed for single NCs after applying the compressive force. Accordingly, our results are strong evidence that the splittings seen in previous work are not due to multiple allowed transitions within individual particles. The force cycle data as well as the spectral and time-resolved data were evaluated with self-written MATLAB routines (Mathworks Inc.) to handle the extensive data sets. For each

Figure 1. Simultaneously collected fluorescence (left, CFM) and height (right, AFM) images of individual NCs spread on a silanized glass surface. After careful alignment of both microscopes, particles could be located at the same positions. The dark lines (fluorescence intermittency) in the left figure are due to blinking of the NCs. 1560

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Nano Letters cycle the slopes of force, spectral position (emission maximum) and fluorescence lifetime versus time were identified both for the attracting (force build-up) and the retracting phase, respectively. From the resulting six gradients any correlation, e.g., spectral position versus applied force (= spectral shift parameter, SSP), could be drawn. For each force cycle we compared attraction and retraction phases and determined their SSPs. For the cases, where significant spectral shifts could be visually observed, the data showed full reversibility in all cases; i.e., no hysteresis or irreversible modification of NCs was observed. In the following data evaluation, however, only cycles have been selected whose SSPs from both phases were comparable within 70%. Cycles with larger discrepancies between attraction and retraction phases were classified as too noisy and therefore ignored. Eventually, the average from the remaining attraction and retraction phases was used to calculate a single SSP for each cycle. In Figure 3, the final distribution is plotted. 15% of the

Figure 4. Correlation of spectral shift parameters (SSP). All possible pairs of SSPs from at least four cycles of each NC lead to a Pearson correlation coefficient r = 0.42.

reproduce the experimental findings. That is, e.g., a blue shifting NC will show cycles with red shifts with 20% probability. These disturbances were chosen independent from the cycle position within a series of cycles as it was found in the experiments. The correlation depicted in Figure 4 illustrates that a single NC often retains its sign of the SSP in repeated cycles. Nevertheless, there are an appreciable number of instances where the sign of the SSP changes, and in addition, even for cases where the sign is conserved, the magnitude of the SSP occasionally exhibits large fluctuations. The most likely reason for this kind of behavior is that, once the AFM tip did not hit a NC at the center, the force induced compression could be accompanied by a rotation of the NC. Thus, in subsequent cycles the force would interact on a different crystal axis with a different shifting behavior. Our findings with respect to the direction and magnitude of spectral shifts suggest an interpretation in terms of bandgap modifications depending on the compression direction relative to the orientation of the crystal axes.19 Bandgaps calculated for pure CdSe NCs as a function of uniaxial compression along the a-axis or c-axis have shown red shifts or blue shifts, respectively. Below 2 GPa biaxial compression (ac-axis) led to blue shifts, while triaxial (hydrostatic) compression always resulted in a blue shift of the bandgap.19 Our observed distribution of 33% red shifts and 67% blue shifts is attributed to sampling a multitude of NC orientations on the glass surface. While the perfect uniaxial compression of particles with a hexagonal wurtzite lattice along the a-axis or c-axis should result in twice as many red shifts as blue shifts, our findings suggest rather biaxial compressions with varying contributions of the principal axes as one would expect for randomly oriented NCs.19 To compare the magnitude of spectral shifts to pressure dependent ensemble optical studies, applied force had to be translated into pressure units. Here we have employed the Derjaguin−Müller−Toporov (DMT) model41−43 to estimate the contact area between tip and NC. Using a tip radius of 7 nm and bulk elasticity properties from CdSe19,44 and Si,43 an applied force of 0−4 nN translates into a pressure of 0−3.8 GPa (see the Supporting Information S5 for details). Although only a first estimate, our experimental setup thus allowed for pressures far above what previous experiments on single chromophores permitted.37 Averaging the data shown in Figure 3, we obtain SSPs of −3.5 meV/GPa and 3.0 meV/GPa for red and blue shifting species, respectively. The calculations for CdSe mentioned above19 gave blue and red shifts of similar magnitude for compression of the c-axis and a-axis, respectively.

Figure 3. Distribution of spectral shift parameters (SSP) obtained by averaging force build-up and retraction phase from each cycle. The bar chart represents the filtered distribution by considering only those cycles whose force buildup and retraction phase show similar SSP within 70%.

recorded force cycles passed our similarity criterion corresponding to 286 cycles from 64 NCs. The distribution displayed in Figure 3 shows a significant asymmetry: 33% of the force cycles are accompanied by spectral red shifts and 67% by spectral blue shifts; i.e., emission energy shifted either to lower or higher values with applied force. Since earlier experimental and theoretical studies of NCs under nonhydrostatic conditions had indicated a dependence of the spectral shifts on the NC orientation, which may change because of the interaction with the AFM tip,19,20 it is important to check whether a NC persists in its shift direction in successive force cycles; i.e., a red shifting NC remains shifting red, and a blue shifting NC remains shifting blue, respectively. As a prerequisite only data with multiple successful force cycles could be evaluated. In the following we therefore considered data from all NCs which passed our filter and where at least four valid cycles (= 4 SSPs) per NC had been identified. A simple correlation plot presenting all possible pairs of SSPs of all NCs considered is shown in Figure 4. The correlation coefficient r = 0.42 indicates a significant stability of the shift direction from a NC under compressive force. To additionally quantify the amount of correlation in a more descriptive manner, we have performed Monte Carlo simulations to mimic our experimental data sets. According to these simulations, a disturbance of the shift direction with a probability of 20% will 1561

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Nano Letters Our findings agree qualitatively with this calculation in the sense that average blue and red shifts are of similar magnitude. A quantitative comparison, however, is prohibitive because of differences in core size17,19 and the existence of a CdS/ZnS shell in our particles. Using a nonhydrostatic pressure medium in a diamond anvil cell, Choi et al.20 reported redshifts for a fraction of CdSe/CdS NCs which had a similar magnitude (5.8 meV/GPa) as our findings. In these experiments, however, it was not clear to which extent the results had been influenced by hydrostatic contributions in the nominally nonhydrostatic setting. Since in our approach a highly directional force is applied, a direct comparison appears to be difficult at this stage. In a recent study on tetrapod nanocrystals embedded in polymer films, significantly higher shift parameters on the order of a few meV/MPa have been reported.45 It was suggested that the tetrapod arms act as antennas that amplify the applied stress in their cores. Interestingly, it was found that at high packing densities of the particles the results were strongly influenced by interactions of the arms from different tetrapods. As mentioned before, the spectral shifts did follow the applied force after mechanical contact between the AFM tip and the NCs. Besides the mechanical interaction also electromagnetic effects due to the tip could potentially influence the fluorescence behavior of the NCs. Typical phenomena reported in the literature are variations of fluorescence intensities and lifetimes with a subtle dependence on various parameters as tip−sample distance and the tip material and shape.46−50 Since the AFM tips used in our experiments are not metallic but made of silicon, we did not expect significant electromagnetic interactions. Nevertheless, we have measured fluorescence lifetimes from the NCs in addition to the emission spectra. Occasionally, we have observed lifetime fluctuations while the AFM tip was approaching a NC. No clear trends, however, could be inferred from these measurements. Moreover, after mechanical contact between the AFM tip and a NC, once force induced spectral shifts were accompanied by alterations of the fluorescence lifetimes, this also did not occur in a systematic manner. In Figure 5 the impact of the applied force on spectral shift and fluorescence lifetime of the NCs is presented in a correlation plot. The general observation of negligible correlation can be quantified by a very small Pearson correlation coefficient r = 0.09 ≪ 1. Thus, we are confident that our main observable, the spectral shifts, which always set in af ter mechanical contact and

followed the applied force, are not driven by electromagnetic interactions. A possible contribution to the spectral shifts could be caused by the quantum-confined Stark effect, ascribed to mechanically induced internal electric fields via the piezoelectric effect. However, the quantum-confined Stark effect dominantly leads to red shifts accompanied by broadening of the spectral lines,51 two phenomena we both did not observe. In contrast, considerably more blue than red shifts have been recorded. This strongly indicates that primarily mechanical forces and not internal electric fields dominate the observed spectral shifts. In summary, we have quantified the impact of adjustable compressive stress on the optoelectronic properties of individual core−shell semiconductor NCs. Forces were transduced by the tip of an AFM cantilever. Within a simple model, the applied force of 0−4 nN translates to pressures of 0−3.8 GPa. Thus, we were able to exert forces on single NCs comparable to the capability of diamond anvil cells. Compatible to former ensemble experiments, we have observed NCs showing either blue or red shifts in emission with applied force, while their spectral shape remained single peaked. Line splitting observed in former ensemble experiments could be clearly attributed to two populations of NCs. Furthermore, a substantial stability of the emission state could be observed; i.e., blue-shifting NCs shifted blue, and red-shifting NCs shifted red in succeeding force cycles. In general, we attribute different responses to distinct orientations of the NC on the glass substrate such that the applied forces act on different axes of the single NCs.19 By monitoring fluorescence lifetimes, electromagnetic interactions between AFM tip and NC could be mainly ruled out as a source for spectral shifts. A further improvement of the experimental setup could involve polarization resolved fluorescence detection.52 Then mechanically induced rotations of a NC can be identified, the latter being a potential source of apparent changes of the shift direction and magnitude. The correlative microscopy approach presented here could also be used to study individual tetrapod nanocrystals for which extraordinarily large spectral shift parameters have been reported.45 By applying compressive force selectively to the different arms, novel insights into the origin of the strong spectral shifts might be obtained. The capability to control the emission wavelength of single semiconductor nanocrystals via the applied force offers further interesting and challenging perspectives. One can imagine a single photon source,53 the emission wavelength of which can be easily tuned within a range of several tens of nanometers. Along another track, applying a compressive force to one NC within an individual NC dimer,54 the transition energy of both particles can be brought into resonance leading to coherent coupling,55 which otherwise is difficult to accomplish because of differences in transition energies due to NC size and static disorder.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b04689. Details of the experimental setup, structural and spectroscopic characterization of NC, and estimation of force-to-pressure conversion (PDF)

Figure 5. Correlation of spectral shift parameters (SSP = dE/dF) with fluorescence lifetime variations (dτ/dF). The Pearson correlation coefficient yields r = 0.09. The lines depict the mean values ⟨dE/dF⟩ = 1.3 meV/nN and ⟨dτ/dF⟩ = −0.2 ns/nN. 1562

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Gerald Hinze: 0000-0003-4355-9325 Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acs.nanolett.6b04689 Nano Lett. 2017, 17, 1559−1563